Fractional distillation of coal tar



June 11, 1957 H. RATTE ETAL FRACTIONAL DISTILLATION OF COAL TAR Filed May 26, 1954 United States Patent O FRACTIONAL DISTILLATION' OF COAL TAR Heinrich Ratte, Frankfurt amV Main, and Julius Geller, Bad Homburg vor der Hohe, Germany, assignors to Rutgerswerke-Aktiengesellschaft, Frankfurt, Germany Application May 26, 1954, Serial'No. 432,354

13 Claims. (Cl. 202-52) This invention relates to a method of fractional distiilation of coal tar and ithas particular relation to a method of separating coal tar by continuous fractional distillation into desired volatile ingredients and a substantially non-volatile distillation residue.

In the processing of coal tar or similar hydrocarbon mixtures by fractional distillation, particular ydifficulties have' been encountered in the separation of phenanthrene-i-anthracene and carbazol, as the vapor pressures of these compounds do not differ much from each other. Furthermore, as anthracene and carbazol .show substantially similar solubility in organic solvents, it is likewise diilcult to separate a mixture of these compounds by dissolution in organic solvents and subsequently crystallization. In .spite of this diii'lculty, hitherto separation of anthracene and carbazol has' been preferably carried out by this process, although, even when proceeding in several steps, the yields are relatively low and the process hasA also other disadvantages, such as the required' heat-consuming redistillation of oil obtained in centrifuging, and the necessity of using extensive apparatus.-

The main difficulty encountered in separating by fractional distillation fractions containing phenanthrene+ anthracene and carbazol, consisted in the necessity of using temperatures, atwhich considerable decomposition of the material to be distilled, occured. For example, in the separation of the relatively low-boiling uorene fraction from higher boiling tar ingredients, the use of a bottom temperature of 310 to 330 C. at a pressure of 150 to 250 mm. Hg is necessary. In view of the high number of trays in the fractionating column to4 be used, it is not possible to economically apply a stillhigher vacuum. On the other hand, as the use of temperatures above 330 C. in the distillation residue for a longer period of time, results in considerable thermal decomposition, the fluorene fraction has to be considered the highest-boiling coal tar ingredient, which could be recovered by direct distillation from the residue, with the use of a high redux ratio.

The main object of the present invention is to provide a method of fractional distillation, in which the high-boiling ingredients of coal tar canA be separated with high reflux ratio, without essentially exceeding for considerable periods of time, aV temperature of about 330 C.

Other objects of the invention are the recovery of individual ingredients with a high degree of. purity and in high yields. Further objects will be apparent from the following disclosure.

It has been unexpectedly found that in continuous rectification of coal tar according to the presentinvention, said peak temperature ofV aboutk 330 C. need not be exceeded considerably and/or for a considerable period of time at any point of thel rectifying plant and the anthracene-pphenanthrene and carbazolfractions can be nevertheless recovered with the` use of highv reux ratios.

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According to the' present invention, this is attained in the followingv manner.

In the iirst rectifying column or in the rst columns of a continuously operated coalv tar distillation plant according Vto theY invention, low-boiling coal tar fractions up to and including the diphenylene oxide fraction, or, if desired, up to and including the fluorene fraction, are distilled offv and the distillation residue of these distillations is freed from the last of said lowboiling fractions by highly efficient rectification' with high reflux ratio in the manner described further below.

This residue, which has been thus sharply defined in its initial boiling point, is subsequently heated and evaporated under highvacuum. The pitch residue resulting' in these steps is continuously discharged, while the vapors formed are passed to one or more, and separated in, rectifying columns. The vacuum and the temperature in the evaporation part of the column are thereby selected in such manner that at least the ingredients up to and including carbazol and eventually also heavy oils, like the pyrene and chrysene fractions, are present essentially in the vapor phase. This is the case if, at an absolute pressure` of l-O to 40 mm. Hg, preferably 20 to 30mm. Hg atl 180 to 220 C. at the top of the column and 20 to 80 mm. Hg. preferably 40 to 60 mm. Hg in the-evaporation part of the column, the tar ingredients are introducedinto this step of the process with a temperature of 350 to 380 preferably about. 360 C;

As this high temperature is maintained for a rather short period of time only, for example 1-3 minutes, it causes no substantial thermal decomposition of the tar ingredients.Y

The ingredients boiling above carbazol, such as primarily ingredients boiling in the range of pyrene and chrysene, can be recovered, ifl desired, directly from the samerectifying column. As in this part of they process particular stress is laid only on sharp fractionation of the carbazol fraction and not' of said ingredients proper, a relatively low reflux ratio and alownumber of normal? trays, or ahigher number of trays with low dipping and adesign favorable t'o ow, can be used, and therefore, the above mentioned' absolute pressure of for example 40'-60 mm. Hg can be maintained without difficulty inthe evaporation space;

After separation of the ingredients boiling above car,- bazol, af fraction of vapors is obtained, at l0 to 40 mm. Hg, corresponding to 180 to 220 C. at the top of the column, which is highly enriched in carbazol, anthracene, phenanthrene and eventually hydroanthracenes and. iluorene. This fraction of vapors is discharged' from the rectifying column and is divided in. a' subsequent group of columns', under a1 lower vacuum, corresponding for example to: -3001mm. Hg, preferably about 150- 200 mm'. Hg, into several fractions. As no pitch ingredients' are treated' infsaidv last mentioned group of columns, separation by distillation of the anthracenephenanthrene-carbazol mixture'v can take place at' high re'ux ratio, i. e. with high supply ofheat, and' longer periods of treatment' in the columns; It has been found that a reux ratio of about 5-15 must be used in this fractionation if a sufficiently sharp separationV is desired. At an absolute pressure of 10U-300 mm; Hg, the highest temperature necessary in the l'ast group of columns amountsto about 290'-320 C., i. e. it can be kept substanti'ally below the temperature at which appreciable thermal decompositionoccurs.

Separation of the phenanthrene-anthracene-carbazol mixture can.I take' placein a mainf column` which can be provided with one or more side columns. In thesecolumns intermediate fractions can beseparatedy tromxther mixture, whereby concentration oh the desired-phenanthrene. -1- anthracene and carbazol fractions, can be considerably increased. A better product also results from obtaining the carbazol fraction not only in liquid form from the sump of themain column, but also in vapor form from the third or dfth tray above the sump, with subsequent condensation, whereby only the heavy oils consisting substantially of residual carbazol, and residual `portions of the pyrene and chrysene fractions, are exposed to thermal stress in the sump of the main column.

Another way of protecting carbazol from thermal decomposition consists in that the ingredients boiling higher than carbazolwith the exception of pitch-are not separated from the carbazol before the last mentioned fractionation. In this case carbazol is separated from said vhigher boiling ingredients and recovered in a subsequent added column as the head product.

ln addition to a fraction containing at least about 70-90% carbazol, the process of this invention yields a well rectified mixture of at least 70-S0% anthracene and phenanthrene, from which the individal compounds can be separated easier than from analogous mixtures obtained in prior processes.

Another advantage is the higher yield, i. e. at least 90-98%, of the individualtar ingredients, resulting from elimination or considerable reduction of thermal decomposition.

A best mode of carrying into etfect the method and the device according to the present invention are illustrated by way of example and without limitation by the following example and the appended drawing which diagrammatically illustrates a system or plant for carrying out the invention.

It is within the scope of the present invention to carry out the functions of the side columns shown by way of example, in the drawing, in normal columns. The term column group is used in this application to denote any of the main columns I, II, Ill and the side columns connected therewith, as well as modiiied arrangements in which main columns are substituted for the side columns shown for performing the functions of the side columns.

Example Coal tar, which has been partially freed from water by evaporation in conventional manner and freed by distillation from other low-boiling ingredients, for example up to and including the naphthalene fraction and if desired also the diphenyl fraction, is fed with a temperature of about 250 C. through pipe 1 to charging tray 2 of rectifying column I, which contains for example a total of trays, in its lower part. Heat is supplied to coil 3 of column I in such amount that the ingredients boiling lower than for example phenanthrene are substantially vaporized and thus separated from the unvaporized residue which contains practically all phenanthrene and other ingredients having equal and higher boiling points.

Redux corresponding to the head product is supplied from the condenser, not shown, of column I through pipe 5 to column head 6 and the amount of redux is so adjusted that the temperature in sump 4 remains practically constant at about 330 C. Absolute pressure in the sump of column I is maintained at about 250 mm. Hg. The temperature at the head of column I is maintained at about 205 C. by adjustment of the redux. A redux ratio of about 2.5-3.5 in column I has been found to give satisfactory results.

Under these conditions, the residue in sump 4, contains only a small amount, such as 2-5% of ingredients boiling below phenanthrene and it contains practically all of the ingredients boiling in the range of phenanthrene and at higher temperatures. Acenaphthene fraction is discharged at the head of column I, while a diphenylenoxyde fraction and a duorene fraction are obtained as side products as indicated in the drawing.

The residue discharged from sump 4 is heated in a heater, e. g. a tube furnace 7, to 350 to 380 C., preferably about 360 C., and introduced to tray 8 in main rectifying column II, which contains e. g. 20-30, preferably 25, trays and is operated at tray 8 under an absolute pressure of 20 to 80, preferably about 40 to 60 mm. Hg. By regulation of the redux supplied through pipe 11, a constant temperature of for example about 180 to 220 C. at l0 mm. to 40 mm. Hg, preferably 210 C. at 30 mm. Hg, is maintained at the head 12 of column Il so that at a corresponding redux between head 12 and the lowest tray 13 for side discharge, the vapors discharged at the head of column are substantially freed from ingredients boiling higher than carbazol and preferably contain practically all carbazol, phenanthrene and anthracene. Said ingredients boiling higher than carbazol amount, for example, to 2-5% in the carbazol-phenanthrene-anthracene fraction discharged at the head of column Il. A redux ratio of about 2.5-3.5 in column II has been found to give satisfactory results.

In column II chrysene and pyrene can be also recovered in side columns 9 and 10, respectively, unless it is preferred to separate these fractions at last from the carbazol as mentioned above. These side columns are operated in such manner that the sumps of these columns are supplied with about the fourfold of the amount of heat necessary for vaporizing the fractions to be recovered from them.

Pitch is discharged at 4' from column Il, as indicated in the drawing.

The vapors discharged at the head of column Il, which in addition to the above mentioned small amount of ingradients boiling above carbazol, contain phenanthrene, anthracene, carbazol, and ingredients boiling within the boiling range of these compounds, are passed to condenser M and from there to the column group III, respectively to the charging tray 8' of column III, part of the condensed liquid being supplied as redux through pipe 11 to column Il by pump P. Column III contains, for example, 40-80, preferably 60 trays. Heat is supplied to sump 15 in an amount sudicient for maintaining a redux ratio of e. g. 10.

The highly enriched carbazol fraction is discharged at 4 while the anthracene-phenanthrene fraction is obtained at the head of column III. 16 indicates the tubing for the redux which corresponds to the head product of column ill. Column lll is operated under an absolute pressure of, for example, mm. Hg whereby the temperature in sump 15 is maintained at about 300 C.

An intermediate fraction containing ingredients boiling between the carbazol fraction and the phenanthrene-Panthracene fraction is obtained in side column 17. The sump of this column must be intensively heated so that, based on the product to be recovered from this side column, at least tenfold, and maximally thirty-fold revaporization take place in order to separate the intermediate products, as much as possible and economical, from anthracene and phenanthrene.

In carrying out the invention, it is necessary to operate the column, in which the anthracene-phenanthrene-carbazol fraction is evaporated (column Il in the drawing) and the column, in which carbazol is separated from anthracene-j-phenanthrene (column lll in the drawing), as well asthe side columns connected with said columns, under vacuum, preferably operating the penultimate column (column II in the drawing) under the highest vacuum.

However, in order to eliminate thermal decomposition as much as possible, it has been found to be of advantage to operate also additional columns, or some of them, in which ingredients boiling lower than phenanthrene are treated, likewise under partial vacuum` It will be understood from the above that, in carrying out the invention, by suitable selection and/or adjustment of the vacuum in the individual columns, the shifting of boiling points of the individual tar ingredients can be extensively utilized for regulating and increasing concentration of the various fractions, particularly of the afec-,eas

. phenanthrene-l-anthracene fraction, and'. of; tle intermea diateI fraction.

Thus, in carrying out the presentV inventiontar, which hasbeen freed from the above mentioned lowest boiling ingredients, is subjected to continuous fractionationin a novel manner with high supply of heat to fractionation and high reiiux, in' separating the tar by rectification from ingredients boiling up to and including. diphenylene oxide, or, if desired, up to and including the. liuorene fraction, in: a rst part` of theprocess; heating the resulting distillation residue to 350 to 380 C. in order to vaporize (under vacuum) high-boiling ingredients including ingredients at least up to phenanthrene, anthracene and carbazol, while continuously discharging distillation residue, in a second part of the process; and subsequently separating the condensed vapors resulting from said second part, by fractionation under vacuum into at least concentrated carbazol, substantially free from phenanthrene and anthracene, and a phenanthrene-l-anthracene fraction which is substantially free from carbazol'.

It has been found that by operating the distillation and fractionation in the above described novel manner, heating to the high temperature of 350-380 C. for a short period of time only,.is suflicient and can be applied with*- out the danger of thermal decomposition. Moreover, owing to the novel character of the fractionating procedure, an unusually high amount of heat canb'e introduced into and a correspondingly high reflux applied to fractionation, likewise without the danger of thermal decomposition. The mixture of phenanthrene, anthracenel and carbazole is obtained in high concentration and substantially free from ingredients boiling above carbazol and this considerably facilitates separation of carbazol from said mixture.

It will be'undersood from the above' that this invention is not limited to the embodiments described above and illustrated in the drawing and can be carried out with various modifications. For example, column` I of the drawing can be operated in such manner'that the iiuorene remains substantially in the residue formed in the sump of column I. In this case iluorene will be present in the phenanthrene-anthracene-carbazol fraction obtained at the head of column II of the drawing and can be recovered by an additional column forming part of column group III. Furhermore, the number and arrangement of side columns can be different from those shown. in the drawing. These and other modifications can be made without departing from the scope of the` invention, as defined in the appended claims.

Reference is made to our co-pending application Ser. No. 191,869, tiled October 24, 1950, and-now'abandoned, of which this is a continuation-in-part.

In carrying out the present invention, the vacuum at the head of column I of the drawing amounts, for examplev to 100 to 200 mm. Hg and this column is operated with a reiiux ratio of, for example, 2`.5` to 3.5;. The amount of heat supplied to the material in heater 7 and in the heating coils of the side columns 9 and 10 is sulicient to bring about a reflux ratioY of, for example, 2.5 to 3.5 in column II. In the sump of column II the temperature is for example. 3.20 to 350' C. and the vacuum corresponds, for example, about 20 to 80 mm. Hg. The condensed fraction fed to tray 8 in column III has a temperature of, for example, 270 to 310 C. The temperature at the head of column III amounts, for example, to 250 to 290 C., and enough heat is supplied to thisV column to maintain, for example, a reflux ratio of 5-15 therein. The vacuum corresponds, for example, to 100 to 300 mm. Hg at the head and, for example, to 200 to 400 mm. Hg in the sump of column III. The temperature of the liquid fed to side column 10 and the corresponding temperature at the head of this column is, for example, 200 to 240 C. and in the sump thereof, for example, 330 to 390 C., While the vacuum corresponds, fon exampleto15` to 45 mm. Hg at. the head and to 30 to90mm. Hg in the sump of column 10. Column 9` is operated, for example, with a feed temperature of about 220.260 C., a temperature atY the head of about 220 to 260 C. and. of 400 to 450 C. in the sump and withfa vacuum of 15 to 501 mm.. Hg at the head and 30 tol 100- mm. Hg inthe sump. Both columns 10 and 9 contain, for example, 10 to 20 trays. Column 17 is operated, forexample, with a feed temperaturev and a temperature atl the head of. 260 to 300 C. and of 285 to 315 C. in the sump and with a vacuum of 125 to 325 mm. Hg attlre head and of 225 to 425 mm. Hg in the sump. It contains, for example, 20 to 40 trays. A pyrene fraction of, for example 60% and a chrysene fraction of, for example, 40% is obtained from columns 10 and 9; respectively and the intermediate fraction discharged from column 17 has, for example-,.a boiling range of 341352 C.

kThe side columns are connected with. a main column and part of the liquid iiowing downward in the main column isfe'd' at the desired level as a side stream into the side column and is' there subjected to fractionation with. a suitable reflux. ratio in. order to obtain at the sump of the side column a fraction boiling at a higher temperature than the fraction escaping at the head of the main. column. The head product of the side column is` reintroduced into the main. column. Byv supplying heat to thesump ofsuchl side column, the amount of heat introduced into the system of the main column can be increased;

The pitch discharged at 4: from column Il is, for example, either normal pitch. having. a softening point of about: 70 C. or ahard pitch. having a softening point of about 901-10 C. Thek headl productl of column. III contains, for example 70-80% of anthraceneal-phenanf threne.

The. term reflux ratio is used in the present application to denote the ratio between Ythe amount ofv reflux and distillate,Y respectively, into which the condensed head vapors. are. divided. The terms diphenylene oxide fraction, acenaphtheneV fraction, iiuorene fraction, pyrene vacuum pump a washing device or a low temperature trap isarranged in order to avoid loss of material,

The pyrene and chrysene fraction can be recovered not only as side product from column II, as described above and shown in the drawing, but also together with the Vanthracene isv facilitated.

head product of column Il. In this latter case, pyrene and chrysene are recovered in the subsequent fractionation, by discharging the carbazol in a side column and the pyrene and chrysene fraction as the sump product of column III. Or a further column can be added, in which carbazol is` obtained as head product and pyrene and chr-ysene as sump product.

The method of the present invention results in essential improvements and advantages, particularly in better separation, higher purity and concentration and also higher absolute yields of the individual products. By the separation ofingredients boiling above carbazol, fractionation of the ingredients boiling between carbazol and In the presence of said ingredi entsy boilingv above carbazol, the partial pressure of the lower boiling ingredients would be reduced and this would result in higher distillation temperatures and, therefore, increased danger of thermal decomposition. Furthermore, by regulation of the reux ratio at the head of column II, the degree of separation between the carbazol fraction and higher boiling: ingredients can be considerably improved by the method of the invention. This separation can be further improved by introducing considerable additional heat through the sumps of columns 9 and 10. Separation of the ingredients in column I can be regulated and improved by corresponding regulation of the amount of heat supplied to sump 4. Finally, in the fractionation step in column IH the amount of heat applied can be increased through the sump of columns Ill and 17 far above the limits hitherto known, whereby separation and concentration of the ingredients is further improved.

The term normal column is used herein to denote a conventional rectifying column connected with the other columns of the apparatus in series.

What is claimed is:

1. A method of continuous fractional distillation of coal tar, comprising the following steps: (a) separating coal tar by continuous fractional distillation into lowboiling ingredients and a distillation residue substantially free from low-boiling ingredients up to diphenylene oxide in a first fractionation; (b) heating said distillation residue to SSW-380 C. and evaporating therefrom under vacuum in the range of 40-80 mm. high-boiling ingredients at least up to phenanthrene, anthracene and carbazol, subjecting the resulting vapors to continuous rectication under vacuum in the range of 20-40 mm., and continuously discharging a vapor fraction containing at least phenanthrene, anthracene and carbazol, and continuously discharging the distillation residue in a second fractionation; condensing said vapor fraction and subjecting it to continuous rectification with high supply of heat for 2 4 fold evaporation of the products and high reflux equal to 1 3 times the products to be recovered under vacuum in the range of 1GO-200 mm. in order to separate it at least into a residue of concentrated carbazol substantially free from phenanthrene and anthracene and a fractionof anthracene-i-phenanthrene substantially free from carbazol in a third fractionation.

2. A method as claimed in claim 1, in which, in the first fractionation, the low-boiling ingredients include also iluorene and, in the second fractionation, the high-boiling ingredients include also pyrene and chrysene and the latter are separated from the vaporized anthracene-phenanthrene-carbazol mixture by taking oli a side stream with a feed temperature of about ZON-240 C. in order to separate pyrene and a second side stream with a feed ternperature of about 220-260 in order to separate chrysene in the second fractionation zone.

3. A method as claimed in claim l, in which at least part of the low-boiling ingredients is distilled off under partial vacuum, in the iirst fractionation.

4. A method as claimed in claim l, in which the residue from the first fractionation is heated to about 360 C. prior to the introduction of said residue into the second fractionation.

5. A method as claimed in claim l, in which coal tar freed from low boiling ingredients up to the diphenyl fraction is used as starting material.

6. A method of continuous fractional distillation of coal tar, comprising the following steps: (a) separating coal tar by continuous fractional distillation into lowboiling ingredients up to diphenylene oxide and uorene, and a distillation residue substantially freed from these low-boiling ingredients, at least part of this fractional distillation being carried out under partial vacuum in a first fractionation; heating said distillation residue after its discharge from said fractionation to about 360 C. and introducing the heated residue into a second fractionation, in which it is subjected to rectification under an absolute pressure of about 20-80 mm. Hg in order to distill ofi at least substantially all phenanthrene, anthracene and carbazol from said residue, subjecting; the resulting vapors to continuous rectification under said absolute pressure and continuously discharging the pitch fOrmed l8 as distillation residue in this fractionation; condensing the rectified vapors containing at least phenanthrene, an- -thracene and carbazol and subjecting the condensate to continuous rectification in a third fractionation under an absolute pressure of 10G-300 mm. Hg, in order to separate it at least into a residue of concentrated carbazol substantially free from phenanthrene and anthracence and a fraction of anthraCene-I-phenanthrene, which is Substantially free from carbazol.

7. A method as claimed in claim 6, in which a reflux ratio of 2-3 is used in the second fractionation and a reiiux ratio of 8-15 is used in the third fractionation.

8. A method as claimed in claim 6, in which, in the third fractionation, the condensate of the rectified vapors is separated into a residue of concentrated carbazol substantially free from phenanthrene and anthracene, a fraction of anthracene-i-phenanthrene substantially free from carbazol, as the lowest boiling fraction of this rectification and an intermediate fraction boiling between the carbazol fraction and the anthracene-l-pbenanthrene fraction and having a boiling range of about 341-352 C.

9. A method as claimed in claim 6, in which, in addition to phenanthrene, anthracene and carbazol, the pyrene fraction and chrysene fraction are also vaporized in the second fractionation and the fraction containing phenanthrene, anthracene and carbazol, is discharged as head product, while the pyrene fraction, as well as the chrysene fraction are branched off as side streams which are subjected to further rectification in the second fractionation zone.

l0. A method as claimed in claim 6 in which the fraction introduced into the third fractionation is separated into a residue of concentrated carbazol substantially free from phenanthrene and anthracene, a fraction of anthracene phenanthrene substantially free from carbazol, as the lowest boiling fraction of this rectification and an intermediate fraction boiling between the carbazol fraction and having a boiling range of about 341- 352 C. .and the anthracene phenanthrene fraction, the fraction of anthracene -lphenanthrene being discharged as head product and the intermediate fraction is branched off as side stream and subjected to further rectification in the third fractionation zone.

ll. A method of continuous fractional distillation of coal tar, comprising (a) separation of coal tar into low boiling ingredients up to diphenylene oxide and into a first residue substantially free from said low-boiling ingredients in a first fractionation; (b) continuously discharging said residue from said first fractionation, indirectly heating it to 350-380 and subsequently introducing it into a lsecond fractionation under vacuum in the range of 20-40 mm., in which it is continuously separated into a vaporized head fraction containing phenanthrene, anthracene `and carbazol, a first side stream consisting of a pyrene fraction and a second side stream consisting of a chrysene fraction, and a pitch residue containing the non-volatile ingredients, all of Said fractions and residue being continuously discharged from said second fractionation zone; (c) continuously introducing the phenanthrene-anthracene-carbazol fraction into a third fractionation under vacuum in the range of 10G-200 mm., in which it is continuously separated into a vaporized head fraction containing phenanthrene anthracene and being substantially free from carbazol, a distillation residue consisting of concentrated carbazol substantially free from anthracene phenanthrene and a side-stream consisting of an intermediate fraction, boiling between the phenanthrene-i-anthracene fraction and the carbazol fraction and having a boiling range of about 341 352 C.

l2. An apparatus for continuous fractional distillation of coal tar, comprising at least one fractionating column for the removal of low-boiling ingredients, said column being adapted to be operated under partial vacuum; a fractionating column containing 20 to 30 trays, and being provided with two side columns; and a furtlher fractionating column containing 40 to 80 trays and being provided with at least one side column, said last mentioned two columns and their lside columns being adapted to be operated under vacuum, the removal of low-boiling ingredients and said rst and second fractionating columns of the apparatus being connected in series; and a heater for heating the residue discharged from the fractionating zone for the removal of low-boiling ingredients.

13. An apparatus as claimed in claim 12, in which normal columns are used instead of the side columns.

References Cited in the le of this patent UNITED STATES PATENTS 1,742,258 Kahl Jan. 7, 1930 2,066,386 Bergeim Jan. 5, 1937 2,175,817 Slawson Oct. 10, 1939 2,594,352 Schmalenbach Apr. 29, 1952 OTHER REFERENCES Coal and Coke Chemicals by Wilson and Wells, 1st ed. 1950 McGraw-I-Iill, Chapter 12, pages 372-399. 

1. A METHOD OF CONTINUOUS FRACTIONAL DISTILLATION OF COAL TAR, COMPRISING THE FOLLOWING STEPS; (A) SEPARATING COAL TAR BY CONTINUOUS FRACTIONAL DISTILLATION INTO LOWBOILING INGREDIENTS AND A DISTILLATION RESIDUE SUBSTANTIALLY FREE FROM LOW-BOILING INGREDIENTS UP TO DIPHENYLENE OXIDE TO 350*-380*C. AND EVAPORATING THEREFROM UNDER VACUUM IN THE RANGE OF 40-80MM. HIGH-BOILING INGREDIENTS AT LEAST UP TO PHENANTHRENE, ANTHRACENE AND CARBAZOL, SUBJECTING THE RESULTING VAPORS TO CONTINUOUS RECTIFICATION UNDER VACUUM IN THE RANGE OF 20-40MM., AND CONTINUOUSLY DISCHARGING A VAPOR FRACTION CONTAINING AT LEAST PHENANTHRENE, ANTHRACENE AND CARBAZOL, AND CONTINUOUSLY DISCHARGING THE DISTILLATION RESIDUE IN A SECOND FRACTIONATION; CONDENSING SAID VAPOR FRACTION AND SUBJECTING IT TO CONTINUOUS RECTIFICATION WITH HIGH SUPPLY OF HEAT FOR 2-4 FOLD EVAPORATION OF THE PRODUCTS AND HIGH REFLUX EQUAL TO 1-3 TIMES THE PRODUCTS TO BE RECOVERED UNDER VACCUM IN THE RANGE OF 100-200 MM. IN ORDER TO SEPARATE IT AT LEAST INTO A RESIDUE OF CONCENTRATED CARBAZOL SUBSTANTIALLY FREE FROM PHENANTHRENE AND ANTHRACENE AND A FRACTION OF ANTHRACENE+PHENANTHRENE SUBSTANTIALLY FREE FROM CARBAZOL IN A THIRD FRACTIONATION. 